1. Field of the Invention
This invention relates generally to rotary tables. More particularly, the present invention relates to a rotary indexing table utilizing an AC induction motor with a high-resolution positional feedback device.
2. Description of Related Art
Rotary tables, such as rotary indexing tables, are well-known for the accurate positioning of work pieces at work stations for automated operations. Rotary indexing tables typically have a table and an indexer assembly that rotates the table through a predetermined angle for positioning work pieces for sequential automated operations.
Rotary indexing tables have been successfully employed in the field of automated assembly for work stations including pick and place devices, feeder bowls, visual inspections, label applicators, robot arms, adhesive applicators, laser machining and other automated assembly processes. Rotary indexing tables are further well-known in the fields of machining for the accurate positioning of work pieces to receive drilling, boring, tapping, CNC machining, facing, grinding, and other types of machining processes. Other uses for rotary indexing tables include the accurate positioning of work pieces for coating, sterilizing, cleaning, testing and calibrating.
As described in U.S. Pat. No. 5,950,503, rotary indexing tables have also been used in the decorating field for screen printing, hot stamping, pad printing, ink jet printing, impact marking, laser marking, spray painting and other decorative processes. For example, rotary indexing tables are currently employed for multi-color screen printing onto work pieces such as CD's, credit cards, key fobs, etc. Typically, a rotary indexing table supports multiple, equidistantly positioned fixtures. The fixtures receive and support the work pieces during the printing operations. At a first work station, a work piece is automatically positioned onto the fixture. The table then rotates through a precise angle or distance to position the work piece under a first screen printing apparatus. After the printing is completed, the table rotates through the same angle again to position the work piece for receiving a second overlaying screen print image. The indexing process continues until the work piece has received all the required layers of screen printing and is removed from the fixture at a final work station.
With the need for very precise machining and close tolerances in manufacture, rotary indexing tables have had to be much more precise and provide more through-put in order for the industry to remain competitive. Rotary indexing tables, for example, may be required to move through a complex set of rotary profiles such as continuous rotation, indexing with a dwell time, oscillation, variable speed or reverse direction. It would be advantageous to have an assembly capable of all these motions while maintaining precision. In addition, with the advent of robotics these assemblies are required to place a work piece at various work angles relative to the work station to provide access from automated operational equipment.
Typically, prior art rotary indexing tables, also known as turntables, are centrally driven and work is performed at the periphery of the table. Alternately, when tables are driven on their outside diameter, the drive mechanism tends to be outside the periphery of the table and thus impedes use of the assembly in various angles and in operations where space is at a premium.
Most prior art rotary indexing tables are driven by cams or geneva mechanisms through a speed reducer and electric motor. Rotary indexing tables of this type suffer from various drawbacks including a fixed number of index positions, the inability to provide continuous rotation and the inability to be programmed.
Another prior art method of driving a rotary indexing table utilizes a ring gear and pinion arrangement powered with a speed reducer and electric motor. This method, however, also suffers from a variety of drawbacks. For instance, the use of a ring gear and pinion arrangement has a lower precision due to backlash. Also, such arrangements are very costly.
A third prior art method of driving a rotary indexing table is through the use of a servomotor configured to drive a cam or pinion gear. The use of a servomotor is costly and also requires a large number of mechanical components. Furthermore, servomotors usually require a load-to-motor inertia mismatch that is very low, such as 10 to 1. If the load-to-motor inertia mismatch exceeds this requirement, the result is instability and poor performance.
A final prior art method for driving a rotary indexing table is the use of a low speed direct drive permanent magnet motor. Such direct drive permanent magnet motors, however, are very expensive. Furthermore, the bearing loads of such motors limit the use of overhung loads on the tooling ring.
Accordingly, a need exists for a rotary indexing table with a drive mechanism that is of low cost while still providing high precision. A further need exists for a rotary indexing table with a simplified design including few moving parts so as to reduce backlash. A final need exits for a drive mechanism for a rotary indexing table that provides accurate positioning and smooth motion in the presence of very high inertial loads.